Splatoon 2 makes sense, considering its success in Japan

Super Mario Galaxy 3 will be in there. It's the most likely game to be the major showpiece for the NX launch.

So you expect in less than a year and a half, Nintendo to release 3 major Mario games for the same platform? Mario Kart, Mario Maker, and 3D Mario will certainly be more spaced out than that, unless this thing gets into trouble and its a handheld.

Splatoon: NX Edition would be my guess

Actually I wouldn't be surprised to see Super Smash Bros. for NX  (including all DLC) and Diddy Kong Racing 2 (until MK 9) too

and a 2D Mario of course, we all know Nintendo

Mario Maker 2.0 just one year after Mario Maker Wii U!?

I am almost certain that Mario 3D will be launch title for NX.

It make sense but not in 1st year if NX will be released in 2016. because Splatoon was released 3 months ago and they still committed to it with updates and new staffs.

But end of 2017. and 2018, very likely.

Why is that odd?

3DS launch-Feb 2011

Super Mario 3D Land-Nov 2011, Mario Kart 7-Dec 2011, New Super Mario Bros 2-July 2012

Wii U launch-Nov 2012

New Super Mario Bros U-Nov 2012, Super Mario 3D World-Nov 2013, Mario Kart 8-May 2014

Both current devices had those within 1.5 years of launch.

I wonder what it could be. It's certainly not Metroid and I really don't see them work on a Diddy Kong Racing game. That's something Monster Games would make!

Despite how people are taking a Q4 2016 launch for granted, I still think Q4 2017 is more likely. So here are two scenarios, based on those two dates and assuming the NX is at least as powerful as the PS4 and X1.

2016 Launch

• Q4 2016 - 3D Super Mario, Zelda U Deluxe
• Q1 2017 - Mario Party 11, Paper Mario 5
• Q2 2017 - Mario Tennis/Golf
• Q3 2017 - Splatoon 2, Super Pokken Tournament
• Q4 2017 - Mario Kart 9, Retro's Secret Project, Mario & Sonic 2018

2017 Launch

• Q4 2017 - Mario Kart 9, Retro's Secret Project
• Q1 2018 - Super Smash Bros 5
• Q2 2018 - 3D Super Mario
• Q3 2018 - Splatoon 2, Hyrule Warriors 2
• Q4 2018 - Super Mario Worlds

Explanations:

• EAD Tokyo released 3D World way back in 2013, so a new 3D Super Mario should be ready early in the NX's life. Failing that, Sunshine and Galaxy 2 were both released in the middle of the year, and Nintendo wouldn't want to release MK9 and this at the same time if they could help it.
• If Zelda is brought to the NX, I think there will be a Wii U version like Twilight Princess was on the GameCube. I'm thinking that the game's main differences would be aesthetic, things like resolution, textures, character models, and music quality. Of course, one exclusive minigame using a minor feature of the NX's control scheme will be implemented.
• Mario Kart games typically have taken three years to develop since MKDS. And of course, this is going to be one of the best selling games on the NX whenever it comes out. The NX needs multiplayer!
• The original Splatoon is a big surprise hit, and one of the best-selling new series from Nintendo in recent years. Especially if you discount handheld games. I suspect that a sequel for the NX would be made a high priority and given extra resources, similar to Melee on the GameCube. Note that Splatoon apparently only took about two years to develop. Sure, EAD Group 2 could also make a minigame collection, but I doubt it.
• "Super Mario Worlds" is what I expect the 2D Super Mario series to evolve into. It would combine the more professionally designed worlds and levels of the NSMB series with the creation tools of Super Mario Maker. Preferably, all the standard levels would be recreatable with the level editor, giving nearly full freedom to players. This game would emphasize the online sharing of content possible on the NX.
• Mario Party 11 is a thing that will inevitably happen. It will either be a late Wii U game or an early NX game.
• Super Smash Bros 5 is not going to be a Sakurai project. I'm sure he'll help out, but Nintendo is not going to be content waiting until early 2021 for the next game in the series. Sakurai himself is not up to making another entry so soon though, though I suspect he would handpick some staff who worked on previous entries. I guess that might be one reason they got Namco to help with the game.
• Hyrule Warriors was a big success for Koei Tecmo, especially in the West. They are not going to sit around and not make a proper sequel incorporating elements of Zelda U.

Unless its a handheld, Nintendo will not launch with another 2D game IMO (excluding a Mario Maker port). 3D Mario will be a superior system seller and I think one will be ready close to launch assuming fall 2016

Zelda is a given. Retro's new game and Animal Crossing are overdue so both a launch window potential (launch-june 2017).

Why you think its not Metroid!?

I don't think Nintendo will give Monster Games to make alone Diddy Kong Racing, very possible with Retro help.